1. Field of the Invention
The present invention relates to an exterior material for a lithium ion battery.
2. Description of Related Art
Lithium ion batteries are widely used as batteries of personal computers, portable terminal devices such as cell phones, video cameras, satellites and the like because they are able to realize thin dimensions, light weight and compact size. In addition, lithium ion batteries are also being actively developed for application to transport machinery exemplified by hybrid vehicles and electric vehicles. Lithium ion batteries are being required to demonstrate performance in terms of larger capacity and lower price while continuing to take advantage of their existing characteristics.
Metal enclosures have conventionally been used for the lithium ion battery exterior materials (to also be simply referred to as “exterior materials”) that house components such as battery cells and electrolytic solution. However, multilayer laminated films are used as exterior materials because of their advantages of light weight, high heat resistance and degree of freedom when selecting battery shape. An example of the configuration of a multilayer laminated film consists of a base material (heat-resistant base material) layer, a first adhesive layer, an aluminum foil layer, a corrosion prevention treated layer, a second adhesive layer and a sealant (heat-fusible film) layer. A lithium ion battery is formed by forming an exterior material composed of a multilayer laminated film into the form of a pouch or deep-drawn formed product by subjecting to deep drawing by cold forming, and filling a battery cell composed of a positive electrode, separator and negative electrode along with an electrolytic solution inside followed by heat sealing. In particular, deep-drawn formed products are widely used since battery capacity increases the greater the draw depth.
The electrolytic solution of a lithium ion battery is composed of an aprotic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate and an electrolyte. Examples of salts serving as electrolytes include lithium salts such as LiPF6 and LiBF4. Since the aforementioned lithium salts generate hydrofluoric acid due to a hydrolysis reaction induced by water, they cause corrosion of the metal surface of the battery cell as well as a decrease in lamination strength between each layer of the multilayer laminated film. Consequently, an aluminum foil layer is provided on the multilayer laminated film of the exterior material to prevent moisture from entering the battery from outside the exterior material.
In addition, exterior materials composed of a multilayer laminated film are broadly classified into two types according to the type of adhesive layer that adheres to the aluminum foil layer and the sealant layer. In other words, these exterior materials are classified as having a dry laminated configuration that uses an adhesive for dry lamination, and a thermal laminated configuration that uses a thermoplastic material in the form of an adhesive resin in the manner of acid-modified polyolefin-based resin (Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2001-202927)). Exterior materials having a dry laminated configuration are widely used in consumer applications such as portable devices requiring moldability and low price. The adhesives used in dry laminated configurations have binding sites such as ester groups or urethane groups that are not sufficiently resistant to hydrolysis (highly hydrolyzable). Consequently, exterior materials having a thermal laminated configuration are used in applications requiring higher reliability. Namely, exterior materials having a thermal laminated configuration are widely used in large-scale applications requiring higher levels of reliability, such as electric vehicles, satellites, submarines or power-assisted bicycles.
More specifically, in the large-scale secondary battery/capacitor market, which includes the automotive field, such as electric vehicles (EV) or hybrid electric vehicles (HEV), and the power storage field, such as lithium ion capacitors (LIC) having the properties of both secondary batteries and capacitors, battery performance is naturally required to have superior long-term stability (10 to 30 years). Consequently, exterior materials having a thermal laminated configuration are used in such applications.
Since lithium ion batteries used in the aforementioned automotive and power storage fields are required to have large electrical capacity in particular, it is necessary to increase draw depth of a multilayer laminated film in particular, and superior deep-drawing formability is required. A known example of an exterior material in which deep-drawing formability has been improved is an exterior material that uses a film base material having specific physical property values as the base material of the outer layer (see, for example, Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2006-228653) and Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. 2006-236938)). However, even if such an exterior material is used, in the case of increasing the draw depth of a deep-drawn formed product, insulation between the electrodes within the battery and the aluminum foil layer may be inadequate.
The following provides a detailed explanation based on an exterior material 110 exemplified in FIG. 3. The exterior material 110 is an exterior material obtained by laminating a base material layer, adhesive layer, aluminum foil layer, corrosion prevention treated layer, adhesive resin layer and sealant layer. For the sake of simplicity, only two layers are shown in FIG. 3 consisting of a first laminated portion 111 consisting of the base material layer, adhesive layer, aluminum foil layer and corrosion prevention treated layer, and a second laminated portion 112 consisting of the adhesive resin layer and sealant layer. Deep drawing of this exterior material 110 is carried out with the sealant layer facing to the inside to form a recess portion 113. At this time, the exterior material 110 is drawn and becomes thin at regions a in near the corners of the recess portion 113 in particular. Consequently, the thicknesses of the adhesive resin layer and sealant layer of the second laminated portion 112 become extremely thin, and insulation between the aluminum foil layer and electrodes within the battery in the first laminated portion 111 may be inadequate when the battery is formed.
In addition, since lithium ion batteries used in the aforementioned automotive and power storage fields are discharged at high current, electrodes and electrode tabs leading outside the battery in order to extract electrical power are extremely large. Consequently, in a lithium ion battery using a deep-drawn formed product formed by deep drawing the exterior material 110, the adhesive resin layer and sealant layer in near an electrode tab 120 are subjected to heat flow and easily become thin when adhering the adhered portion between the exterior material 110 and the electrode tab 120 around the periphery of the battery as shown in FIG. 4, thereby resulting in inadequate insulation between the electrode tab 120 and the aluminum foil layer within the exterior material 110.
On the other hand, at the periphery of the lithium ion battery where deep-drawn exterior materials are heat-sealed, moisture easily enters between the adhesive resin layer and sealant layer at seal end surfaces 114. Consequently, moisture resistance of the seal end surfaces 114 is required to be improved by reducing the thicknesses of the resin adhesive layer and sealant layer in order to prevent hydrolysis of lithium salt and improve long-term reliability. However, when the thicknesses of the resin adhesive layer and sealant layer are reduced in order to improve moisture resistance, electrical insulating properties decrease as previously described. Consequently, it is difficult to simultaneously realize moisture resistance of the seal end surfaces 114 and electrical insulating properties between the aluminum foil layer and the electrodes and electrode tabs of the exterior material.
In addition, another example of an exterior material is known that is obtained by laminating a base material layer, adhesive layer, aluminum foil layer, corrosion prevention treated layer and adhesive resin layer, or in other words, an exterior material in which a sealant layer is not provided on the surface of the aluminum foil layer provided with the adhesive resin layer. In this case, as shown in FIG. 5, in order to form a lithium ion battery by deep drawing an exterior material 110A that does not have a sealant layer and in which the innermost layer is an adhesive resin layer, adhesion between the exterior material 110A and the electrode tab 120 around the periphery of the battery is carried out using a film 130 that adheres to metal. However, even in the case of this type of exterior material that does not have a sealant layer, problems occur similar to those in the case of the aforementioned exterior material 110.
The following two types of packaging forms have been proposed for the form of sealing battery contents consisting of a positive electrode, separator, negative electrode, electrolytic solution and tab composed of a lead and tab sealant and the like for use as the form of a lithium ion battery using a (laminated type) exterior material employing a multilayer laminated film:
(i) pouch type packaging that uses an exterior material to form a pouch that houses the battery contents; and,
(ii) embossed type packaging that subjects an exterior material to cold forming to form a recess portion that houses the battery contents therein.
In an embossed type packaging of packaging form, a form is also employed that increases battery capacity by increasing housing volume by forming recess portions in both sides of the laminated exterior material in order to more efficiently contain the battery contents. For example, as shown in FIG. 11, by housing a positive electrode, separator, negative electrode and electrolytic solution in recess portions 111a of two exterior materials 110a having the recess portions 111a formed by cold forming, and heat sealing so as to sandwich tabs 120a composed of a lead 121a and tab sealant 122a there between, the resulting lithium ion battery 101a is sealed by forming heat sealed portions 112a. 
In the forms described in the aforementioned (i) and (ii), sealing is carried out by aligning the sealant layers of the exterior materials 110a and heat sealing the ends of the exterior materials 110a with the tabs 120a sandwiched there between as in the lithium ion battery 101a exemplified in FIG. 11. Heat sealing is controlled by the three conditions of temperature, pressure and time. In general, when heat sealing at a high temperature, although sealing time can be shortened, if the temperature is excessively high, problems such as resin deterioration are presumed to occur. If the pressure is excessively low, entanglement of the deposited resin decreases, interfacial separation occurs easily, and peel strength decreases. If the pressure is excessively high, the heat sealed portions 112a become thin and peel strength decreases. In addition, resin reservoirs are formed by resin that has been extruded from the heat sealed portions 112a, strain is generated around the heat sealed portions 112a, and there is increased susceptibility to the local application of a load at the portions where strain is generated, thereby causing a decrease in peel strength or resulting in increased susceptibility to separation at locations other than between the sealant layers. In addition, although heat sealing is a short period of time is advantageous in terms of operability and costs, when considering adequate sealing performance, heat sealing is required to be carried out for a minimum fixed period of time.
On the other hand, since electrolytes such as LiPF6 typically used in the lithium ion battery 101a cause deterioration of battery characteristics resulting from the generation of hydrofluoric acid by hydrolysis, it is necessary to reduce moisture permeability from sealed ends 113a. The physical properties of the heat sealed portions 112a of the exterior materials 110a have a considerable effect on moisture permeability from the sealed ends 113a. If the film thickness of the heat sealed portions 112a of the exterior materials 110a is reduced, the amount of water permeating from the sealed ends 113a can be reduced. However, the physical properties of the sealed ends 112a of the exterior materials 110a also have a considerable effect on insulating properties between the leads 121a of the tabs 120a and the aluminum foil of the exterior materials 110a. In other words, if the film thickness of the heat sealed portions 112a is reduced, since the distance between the aluminum foil layer of the exterior materials 110a and the tabs 120a decreases, it becomes difficult to ensure insulating properties between the aluminum foil layer and the tabs 120a. In addition, due to the formation of resin reservoirs, there is also the risk of a decrease in peel strength due to the generation of strain at the interface between the sealant layers around the heat sealed portions 112a. 
A known example of a method for reducing permeation of moisture consists of improving barrier properties by promoting molecular orientation in the sealant layers and the like by drawing. However, this drawing method causes problems such as decreased heat sealing performance or decreased formability.
In addition, Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. H11-086808) indicates an exterior material that prevents corrosion of an aluminum foil layer by hydrofluoric acid by reducing moisture permeation from sealed ends in which a carboxylic acid metal salt or metal oxide, or an inorganic substance such as hydrotalcite or magnesium sulfate, is dispersed in an adhesive resin layer composed of a thermoplastic resin formed between the aluminum foil layer and a sealant layer. In this exterior material, effects are obtained such as absorption of moisture that has entered from the sealed ends and capturing of hydrofluoric acid by the dispersed substance.
However, although the exterior material of Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. H11-086808) is expected to demonstrate temporary effects, it is difficult to sustain those effects over a long period of time, while also having disadvantages such as requiring storage in a dry environment. In addition, since the aforementioned substance is required to a dispersed in a fixed minimum amount in order to obtain adequate effects, it is difficult to make the thickness of the exterior material to be 100 μm or less. Moreover, there is also the risk of increased moisture permeation due to effects such as crystallization of the thermoplastic resin being inhibited by the dispersed substance.
Furthermore, a lithium ion battery is also referred to as a lithium secondary battery, the electrolyte thereof is composed of a solid polymer, gelled polymer or liquid, is a battery that generates electromotive force due to the migration of lithium ions, and includes that in which the positive electrode and negative electrode active materials are composed of high molecular weight polymers.
Lithium ion batteries enabling reduced thickness, light weight and compact size have recently become quite familiar as a result of having been actively marketed as batteries used in applications such as personal computers as well as cell phones and other portable terminal devices. In addition, there has recently been active development of applications to transport machinery exemplified by hybrid vehicles and electric vehicles, and these batteries are being required to demonstrate performance in terms of increased size, increased capacity and lower price while taking advantage of existing characteristics.
The electrolytic solution used in lithium ion batteries is composed of an electrolyte and an aprotic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate. In addition, salts such as LiPF6 or LiBF4 are used as lithium salts used for the electrolyte. However, since these lithium salts generate hydrofluoric acid due to a hydrolysis reaction with water, they caused corrosion of metal surfaces and decreases in lamination strength between each of the layers of multilayer films. Consequently, the entry of moisture has been blocked from the surface of multilayer laminated films by using aluminum foil for a portion of the multilayer film for the purpose of preventing entry of moisture.
This type of multilayer laminated film typically employs a configuration in which a sealant layer is laminated on a first surface (one surface) of an aluminum foil layer through an adhesive layer, and a base material layer is laminated on a second surface (other surface) through an adhesive layer. The adhesive layers are broadly classified into that having a dry laminated configuration composed of an adhesive resin layer for dry lamination, and that having a thermal laminated configuration composed of a thermoplastic material. Since adhesives of dry laminated products have highly hydrolyzable binding sites such as ester groups or urethane groups, a hydrolytic reaction induced by hydrofluoric acid occurs easily. Thus, thermal laminated configurations are used in applications requiring high reliability.
In addition, in the large-scale secondary battery/capacitor market, such as the automobile industry, in which development is proceeding on automobiles using only secondary batteries or those combining the use of gasoline and secondary batteries in the manner of electric vehicles (EV) or hybrid electric vehicles (HEV), or the power storage industry, in which development is proceeding on electrical double-layer capacitors (ELDC) for storing electrical power manufactured in solar cells or wind power generators or lithium ion capacitors (LIC) having the properties of both secondary batteries and capacitors, thermal laminated configurations are used that are capable of ensuring high reliability as previously described for applications in which battery performance is naturally required to have greater safety and long-term stability (10 to 30 years).
Multilayer laminated films are required to have greater deep-drawing formability in response to needs for increased electrical capacity for use in lithium batteries used in such automotive or power storage applications. Moreover, since automotive applications are presumed to involve use in high-temperature climates, lithium batteries are also required to have heat resistance even when used in regions subjected to considerably high temperatures in consideration of installing the batteries around an engine.
In consideration of this background, draw depth is one of the factors that affect performance in multilayer laminated films. As described above, draw depth is extremely important in terms of electrical capacity in lithium battery exterior materials. Conversely, deep-drawn formed products are such that thickness decreases from the original wall thickness at a formed corner D as shown in FIG. 13. In this case, this means that the inner sealant layer is subjected to strain due to drawing at the formed corner D, resulting in the occurrence of whitening, a phenomenon by which the sealant layer becomes white and turbid along the formed site. This phenomenon involves the formation of microcracks at the interface between crystalline portions and amorphous portions due to the effects of crystallization of the maleic anhydride-modified polyolefin resin used for the sealant layer and adhesive resin, and the effects of strain caused by deep drawing, thereby causing the sealant layer to appear white due to scattering of light, and is referred to as void crazing. This whitening phenomenon is also one of the factors causing concern regarding insulating properties with aluminum foil.
In particular, there are many cases in which an acid-modified polyolefin-based resin, which has been graft-modified with maleic anhydride, is used as a film layer that contacts aluminum foil typically used in thermal laminated configurations, on the inside of a multilayer laminated film in thermal laminated configurations. Since acid-modified polyolefin-based resins that have been graft-modified with maleic anhydride alleviate stress generated in the case separation has occurred at the interface between aluminum foil and the polyolefin-based resin graft-modified by maleic anhydride in addition to chemical adhesion using functional groups of maleic anhydride with respect to adhesion with metal foil in particular, physical adhesion is improved by incorporating an immiscible elastomer.
An island-sea structure is formed in which an immiscible elastomer is dispersed in the polyolefin graft-modified by maleic anhydride on the micrometer order, and since well-defined interfaces are formed (FIGS. 14A and 14B), microcracks form at these interfaces leading to a whitening phenomenon due to strain generated during deep drawing in particular.
Another factor is improvement of the above-mentioned heat resistance. Heat resistance as referred to here refers to heat seal strength in a multilayer laminated film. In general, in the case of lithium batteries, electrolytic solution deteriorates due to repeated charge and discharge cycling or high temperature and humidity in the work environment, thus resulting in the generation of pyrolytic gas components. At this time, the generation of this gas is accompanied by an increase in internal pressure, thus requiring heat sealed portions of the multilayer laminated film to have adequate heat seal strength (seal strength). However, the exterior material for a lithium ion battery of the multilayer laminated film type is characterized by having a gas escape mechanism by which, in the case internal pressure has increased due to the generation of gas accompanying deterioration of electrolytic solution in a high-temperature environment, the lamination strength of the lithium ion battery exterior material that has been stored in a high-temperature environment decreases and gas generated accompanying an increase in internal pressure is allowed to escape by utilizing this decrease in lamination strength. Thus, although a decrease in lamination strength in a high-temperature environment cannot always be said to be an area of concern, differing from ordinary consumer applications, since it is necessary for the battery to operate in a considerably harsh environment when placed in the environment associated with the aforementioned automotive applications, heat resistance to a greater degree than that of consumer applications is thought to be preferable.
Numerous publications of the prior art, including those defining the physical properties of the outer layer film base material and those using a thermal laminated configuration in order to improve deep-drawing formability, have been disclosed that describe exterior materials for lithium ion batteries. However, these publications are not recognized to contain details describing improvements with respect to the improvement of the aforementioned whitening phenomenon that occurs during forming or improvement of heat resistance.
On the basis of the above, it is necessary to inhibit a whitening phenomenon during cold forming for applications requiring long-term reliability such as automotive applications and power storage applications which are likely to proliferate in the future.
Lithium ion secondary batteries (to also be simply referred to as “lithium batteries”) enabling reduced thickness and compact size while generating high levels of energy are being actively developed for use as secondary batteries for consumer applications used in cell phones and other portable terminal devices, video cameras and the like. Deep-drawn formed products obtained by cold forming (deep drawing) of a laminated film having a multilayer configuration are used as lithium ion battery exterior materials used in lithium batteries. An example of the configuration of a laminated film having a multilayer configuration consists of a heat-resistant base material layer, aluminum foil layer and sealant (heat-fusible film) layer. In addition, since exterior materials that use such a laminated film not only offer a high degree of freedom with respect to battery shape, but also are lightweight, have high heat dissipation and are inexpensive, they have been attempted to applied to batteries of environmentally-friendly hybrid vehicles and electric vehicles that have demonstrated remarkable growth in recent years.
Lithium batteries using a laminated film type of exterior material are formed by housing battery body components, consisting of a positive electrode material, negative electrode material and separator along with an electrolytic solution obtained by dissolving a lithium salt in an aprotic solvent (such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate) or an electrolyte layer composed of a polymer gel impregnated with this electrolytic solution, in the aforementioned deep-drawn formed product, followed by heat sealing with a heat seal.
The aforementioned electrolytic solution is highly permeable with respect to the sealant layer. Consequently, electrolytic solution that has penetrated into the sealant layer may decrease lamination strength between the aluminum foil layer and sealant layer and ultimately cause leakage of the electrolytic solution. In addition, since electrolytes in the form of lithium salts such as LiPF6 or LiBF4 generate hydrofluoric acid due to a hydrolytic reaction, they cause corrosion of metal surfaces and decreases in lamination strength between each layer of the laminated film. Therefore, Patent Documents 5 to 7 (Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2001-243928, Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2004-42477, Patent Document 7: Japanese Unexamined Patent Application, First Publication No. 2004-142302), for example, disclose exterior materials that are resistant to electrolytic solution and hydrofluoric acid and are resistant to the occurrence of delamination.
In addition, in exterior materials fabricated by dry lamination, the urethane-based adhesive used may be swollen by electrolytic solution thereby causing delamination. Patent Document 8 (Japanese Unexamined Patent Application, First Publication No. 2002-187233) indicates that an exterior material can be fabricated that inhibits delamination by using a dry lamination method that uses a urethane-based adhesive that is resistant to electrolytic solution. However, exterior materials fabricated by thermal lamination are used for large-scale applications requiring high reliability in particular.
Since lithium batteries used in large-scale applications are required to have superior reliability and long-term stability in particular, the level of performance required of exterior materials used in these lithium batteries is also high. Conventionally, since hydrofluoric acid is generated by hydrolysis of electrolyte in the form of a lithium salt, evaluations using water were hardly carried out at all to evaluate exterior materials. However, in the case of lithium batteries for large-scale applications, the environment in which they are used is harsher in comparison with consumer applications. Consequently, it has become necessary to examine evaluations premised on the occurrence of delamination caused by corrosion of aluminum foil due to an increase in the amount of hydrofluoric acid generated accompanying excessive moisture absorption. From this viewpoint, there are a growing number of cases in which water resistance and hydrofluoric acid resistance are being used to evaluate exterior materials. When evaluating an exterior material using an electrolytic solution, normally a sample of the exterior material in the shape of a strip (such as a strip consisting of a heat-resistant base material, aluminum foil layer and heat-fusible film layer) is immersed in electrolytic solution at 85° C. followed by confirming the presence or absence of delamination. Moreover, a method consisting of rinsing the sample with water after immersing in electrolytic solution and then immersing in water has also been proposed in order to also evaluate handling and water resistance. Moreover, an accelerated test has also come to be carried out in which a sample is immersed in electrolytic solution at 85° C. into which water equivalent to several ppm has been dropped in advance in order to evaluate under conditions of excessive generation of hydrofluoric acid.
A method consisting of chemical conversion treatment of aluminum foil is known to be the most effective method for imparting resistance (electrolyte resistance, water resistance and hydrofluoric acid resistance). An example of chemical conversion treatment is chromate treatment. Patent Document 9 (Japanese Unexamined Patent Application, First Publication No. 2002-144479), for example, discloses numerous types of chromate treatment, such as coating chromate treatment or chromate treatment by immersion. Chemical conversion treatment as exemplified by chromate treatment is being examined regardless of whether applying to consumer applications or large-scale applications. In recent years, methods have been examined for carrying out corrosion prevention treatment on an aluminum foil layer without using chromium compounds in consideration of the effects of chromium compounds on the environment. For example, Patent Document 10 (Japanese Unexamined Patent Application, First Publication No. 2007-280923) indicates an exterior material that imparts electrolyte resistance, hydrofluoric acid resistance and water resistance without using a chromium compound.
On the other hand, exterior materials are also required to have superior formability. In other words, since energy density is determined by the extent to which cells and electrolytic solution can be contained within a lithium battery, in order to further increase the capacity thereof, the draw depth is required to be further increased when forming the exterior material into the shape of a battery.
Although forming of the exterior material is typically carried out by drawing with a metal mold, if the forming depth at this time is too deep, cracks and pinholes may form in those portions of the lithium battery exterior material that are drawn by forming, thereby resulting in a loss of reliability of the battery. Consequently, it is important to increase forming depth without impairing reliability.
In large-scale applications such as electric vehicles in particular, although there is a desire to extract a large amount of current in terms of battery performance, since it is also desired to further increase energy density, both superior reliability and long-term storage stability are required simultaneously.
Although exterior materials employing a thermal laminated configuration are able to impart adequate electrolyte resistance even in large-scale applications, it is difficult to obtain adequate formability with respect to cold forming. Consequently, at drawn portions such as formed lateral surfaces and corners in particular, microcracks easily form in the sealant layer of the exterior material due strain generated during cold forming, thereby resulting in increased susceptibility to whitening. In the worst possible case, these microcracks end up joining causing the sealant layer per se to rupture, while further resulting in the risk of additional rupturing of the aluminum foil layer and heat-resistant base material layer. Thus, the occurrence of a whitening phenomenon in an exterior material due to cold forming is considered to be a preliminary warning of potential rupture of the exterior material per se. Consequently, it is obviously important to be able to inhibit a whitening phenomenon to prevent rupture of the exterior material during cold forming even if forming depth is increased.